1. Field of the Invention
The present invention relates to a piezoelectric actuator, an pyroelectric infrared sensor and a piezoelectric light deflector which employ the piezoelectric actuator.
2. Description of Prior Art
Piezoelectric actuators, which make bending motion under the influence of a voltage applied across a piezoelectric element made by bonding a piezoelectric material onto thin elastic plates made of a metal or the like, are used in various devices including a chopper for a pyroelectric infrared sensor and a light deflector. Actuators of this type are classified into bimorph type where piezoelectric ceramic plates are bonded on both sides of a thin elastic plate, and unimorph type where a piezoelectric ceramic plate is bonded on only one side of a thin elastic plate, which are selected and used according to the application. When the piezoelectric actuators of bimorph type and unimorph type of the prior art are required to produce a large amount of displacement, applied voltage is increased or the frequency of the drive voltage is made equal to the resonance frequency of the element.
However, such methods lead to significantly increased strain of the piezoelectric oscillator which makes it impossible to achieve a high reliability.
There has also been such a problem that the element must be larger in size in order to achieve a large amount of displacement while minimizing the strain of the piezoelectric diaphragm.
While the piezoelectric actuator can be driven with a lower voltage by using resonance, this causes the drive section to vibrate with a larger amplitude which lowers the reliability, resulting in another problem that the displacement increases due to variations in the resonance.
For these reasons, it has been difficult for the piezoelectric actuators of bimorph type and unimorph type of the prior art to satisfy the requirements of decreasing the drive voltage, increasing the amount of displacement, improving the stability and reducing the size at the same time.
The pyroelectric infrared sensor, which has recently found wider applications such as measurement of food temperature in a microwave oven and locating people in a room for air-conditioning control, also employ piezoelectric actuators. The pyroelectric infrared sensor utilizes the pyroelectric effect of pyroelectric material such as single crystal of LiTaO.sub.3 which can be explained simply as follows. A pyroelectric material undergoes spontaneous polarization and always has surface charges which, under stationary state in atmosphere, couple with charges in the atmosphere thereby to maintain electrical neutrality. When the pyroelectric material is irradiated with infrared ray, the pyroelectric material changes the temperature thereof thus bringing the surface charges out of the neutrality. The pyroelectric infrared sensor measures the intensity of the infrared ray by detecting the charges generated on the surface. In other words, every object emits infrared ray which corresponds to the temperature thereof, which can be measured with this sensor thereby to determine the temperature or the position of the object.
Since the pyroelectric effect occurs as the intensity of the incident infrared ray changes, the pyroelectric infrared sensor must change the intensity of the incident infrared ray. A chopper is usually used for this means, so that the infrared ray is incident on the pyroelectric material intermittently and accordingly the object temperature is measured. Pyroelectric infrared sensors of the prior art employ mainly choppers which is based on an electromagnetic motor, a piezoelectric actuator, etc.
FIG. 46 schematically shows an pyroelectric infrared sensor of the prior art which employs a piezoelectric actuator comprising piezoelectric ceramics bonded onto a thin elastic plate as a chopper.
In the pyroelectric infrared sensor of the prior art shown in FIG. 46, piezoelectric ceramic plates 311a, 311b are bonded on either side of an elastic shim 310 thereby constituting a bimorph type element. The piezoelectric ceramic plates 311a, 311b have electrodes formed on the surfaces thereof, and are made to polarize in the direction of thickness. Directions of polarization of the piezoelectric ceramic plates 311a, 311b are determined so that the piezoelectric ceramic plates 311a, 311b deform always in the opposite directions. That is, polarity of the applied voltage and the direction of polarization are determined so that one of the piezoelectric ceramic plates 311a, 311b expands while the other contracts. The bimorph type element is supported by a supporting member 313 and has, at the tip on a free end thereof, a shading plate 14 which is located between the incident light and the infrared sensor thereby to interrupt the incident light. The infrared sensor 315 is disposed in the vicinity of the bimorph type element in such an arrangement as the infrared sensor 315 does not touch the shading plate 314 and the bimorph type element.
When a voltage is applied across the elastic shim 310 and the piezoelectric ceramic plates 311a, 311b in the pyroelectric infrared sensor of the prior art made in such a configuration as described above, the bimorph type element makes bending motion while being fixed at one end thereof, while the shading plate 14 attached to the tip makes reciprocal motion as the direction of the electric field changes. The shading plate 314 making the reciprocal motion interrupts light beam 316 incident on the infrared sensor 315.
However, a chopper used in the pyroelectric infrared sensor must make a relatively large displacement. Therefore in the bimorph type piezoelectric chopper of the prior art, a large displacement is achieved by applying a higher voltage and setting the frequency of the drive voltage equal to the resonance frequency of the element, while employing such a construction as the piezoelectric oscillator is directly supported. This construction leads to a problem that the support portion of the piezoelectric chopper is subject to a significant strain which makes it difficult to achieve a high reliability in the support member. There has also been such a problem that the element must be larger in size in order to achieve a great amount of displacement while minimizing the strain of the piezoelectric oscillator.
While the piezoelectric actuator can be driven with a lower voltage by using resonance, this causes the drive section to vibrate with a larger amplitude which lowers the reliability, thus resulting in another problem that the displacement increases due to variations in the resonance.
For these reasons, it has been difficult for the piezoelectric actuators of bimorph type and unimorph type of the prior art to satisfy the requirements to decrease the drive voltage, increase the displacement, improve the stability and reduce the size at the same time.
Recently, in the trend toward sophistication of physical distribution systems, the bar code is extensively used to control the commodities based on digital data. A bar code reader used to read the bar code directs a laser beam to the bar code and detects the pattern of the reflected light, thereby reading the information from the bar code. As such, the bar code must have a mechanism for deflecting the laser beam generated by a laser source. While a deflector based on a 2-pole motor having a reflector have been used, light deflectors based on the piezoelectric effect have recently been put into practical use in order to make apparatuses that incorporate the light deflector smaller in size.
As light deflectors based on the piezoelectric effect, V. J. Fowler & J. Schlafer, Proc. IEEE.,Vol.54 (1966), pp1437 discloses one that comprises an actuator made by laminating piezoelectric elements and attaching a mirror thereto wherein the direction of the mirror is controlled by applying a voltage to the actuator (hereinafter called first light deflector of the prior art).
There are various types in addition to that described above. For example, In Japanese Non-examined Patent Publication No. 58-95710 there is disclosed another light deflector which deflects the direction of light by rotating a mirror by means of a bimorph actuator (hereinafter called second light deflector of the prior art). In Japanese Non-examined Patent Publication No.58-189618 there is disclosed a light deflector having a bimorph actuator with electrodes divided into a plurality of parts which controls the amount of deformation by controlling the number of electrodes whereon voltages are applied (hereinafter called third light deflector of the prior art).
The first light deflector of the prior art has such a problem as the angle of deflecting the light with respect to the applied voltage cannot be increased sufficiently because an actuator of laminated elements is used.
The second light deflector of the prior art has such a problem as extremely complicated construction because a plurality of bimorph actuators and a rotating shaft of the mirror are mechanically coupled.
The light deflector of the prior art has such a problem as a complex process is required to control the amount of deflection.